The invention relates to “active implantable medical devices” as defined by Directive 90/385/EEC of 20 Jun. 1990 the Council of the European Communities. The invention relates to implants for delivering vagus nerve stimulation therapies, which have been called “VNS” therapies (Vagus Nerve Stimulation).
Stimulation of the vagus nerve affects cardiovascular function by reducing the heart rate and myocardial contractility with decreased duration of diastole. These effects can help reduce the progression of cardiac remodeling that may lead heart failure.
In general, the vagus nerve can be stimulated asynchronously or synchronously (e.g., with the heartbeat). In the first case, the device may simply include a lead provided with an electrode implanted on the vagus nerve and of a generator delivering VNS pulses on this electrode. In this configuration there is no possible interference between the VNS electronics and a cardiac lead.
In contrast, in the case of a synchronous stimulation, for example, the device further includes one or more cardiac leads. For example, such a device may include one or more endocardial lead or one or more lead implanted in the coronary network for the collection of the cardiac depolarization waves. Such a device may optionally deliver myocardial stimulation pulses (stimulation of ventricular and/or atrial cavities) using electrodes in the heart, in addition to the VNS stimulation applied separately on the vagus nerve.
U.S. 2012/0303080 A1 and U.S. 2007/0233194 A1 disclose devices for synchronous stimulation of the vagus nerve. In this configuration of synchronous VNS stimulation, pacing should be delivered in a non-vulnerable period of the ventricle.
Particularly, if we consider an electrocardiogram ECG surface or endocardial electrogram EGM, among the different waves representative of the PQRST complex of the cardiac activity, it is known that during the QRS (ventricular depolarization) and the subsequent T wave (ventricular repolarization) the heart is in a refractory period. This ventricular refractory period includes a period called “absolute refractory period” during which no electrical stimulation will have an effect on cardiac cells, followed by a period called “relative refractory period” during which stimulation may excite some heart fibers and induce ventricular arrhythmia.
In the case of stimulation of the vagus nerve and in the case of a system with a ventricular lead, it is commonly accepted that VNS stimulation delivered during the relative refractory period of the ventricle is potentially harmful because charges that could be accumulated on the electrode may trigger ventricular arrhythmias. Stimulation should therefore be avoided during this period. Thus, US 2012/0303080 A1 and US 2007/0233194 A1, cited above disclose synchronizing the application of the VNS pulse burst on the detection of the R wave, in order to apply these pulses during the absolute ventricular refractory period which immediately follows this wave. Despite these prior art devices, it remains challenging and difficult to safely synchronize VNS and heart stimulation treatments.